The DDD(R) pacing mode is often used in patients with Sick Sinus Syndrome (SSS), a term that covers a large array of sinus node disease states. Such patients often have intact AV conduction. If the pacemaker's AV interval (also known as AV delay) is not properly programmed, the pacemaker will deliver an unneeded and undesirable ventricular pacing pulse. More specifically, in the DDD(R) pacing mode, the ventricular sensing channel waits for a conducted beat until the programmed AV interval (AV delay) times out. If an intrinsic ventricular activation is not detected during the programmed AV delay, ventricular pacing is performed. In certain patients, this may result in a higher than necessary percentage of ventricular pacing.
There is growing medical evidence that inappropriate ventricular pacing has disadvantageous short-term hemodynamic effects and may prove harmful when allowed to continue for an extended period of time. For example, studies have shown that ventricular pacing results in asynchronous delayed activation of the ventricular tissue, which compromises hemodynamics in mammals. Canine studies have shown that right ventricular apical (RVA) pacing causes a negative inotropic effect and an undesirable reduction in cardiac efficiency. Additionally, long term RVA pacing has been shown to lead to permanent changes including myofibrillar cellular disarray, myocardial perfusion defects, and structural abnormalities. Each of these may further contribute to deterioration of left ventricular function.
Further studies have suggested that high rate ventricular pacing renders patients more susceptible to the induction of ventricular tachycardia, as compared to high rate atrial pacing with normal ventricular contractions. Such studies, combined with the growing body of evidence showing the detrimental effects of long-term ventricular pacing, has led to more deliberate efforts by clinicians and implantable cardiac device manufacturers to allow for normal ventricular activation when programming dual chamber bradycardia devices.
To reduce the extent of ventricular pacing, medical device manufacturer Medronic has developed an algorithm that it calls “Managed Ventricular Pacing” (MVP), which is believed to have been implemented in both implantable defibrillators and pacemakers. In U.S. Pat. No. 7,130,683, entitled “Preferred ADI/R: A Permanent Pacing Mode to Eliminate Ventricular Pacing While Maintaining Back Support”, which is assigned to Medtronic, the MVP acronym equivalently stands for “Minimum Ventricular Pacing”. The MVP algorithm, which is described in some detail below, is described in more detail in U.S. Pat. No. 7,130,683, which is incorporated herein by reference.
In the MVP algorithm, the AAI(R) mode is the primary mode of pacing. In the AAI(R) mode, sensing occurs in the atrium, pacing occurs in the atrium, and atrial pacing is inhibited if an intrinsic atrial event (i.e., a P wave) is detected within a programmed atrial escape interval (AEI). However, unlike a typical AAI(R) mode of operation, in the MVP algorithm ventricular events are also sensed (i.e., the ventricular channel is monitored), and thus this mode may alternatively be referred to as an ADI(R) mode, or AAI(R)+mode, because there is ventricular backup. So long as a ventricular event is sensed anywhere within a given A-A interval, the pacing mode remains in the MI(R) mode. However, ventricular backup pacing occurs as needed, in the presence of a transient loss of AV conduction. More specifically, when a P wave is blocked (i.e., not conducted through the AV node), resulting in an atrial event not being sensed within the A-A interval, a single ventricular pace is provided, which can be synchronized on the next P wave.
When there is persistent loss of AV conduction, the pacing mode is switched to the DDD(R) mode. For example, if there is loss of AV conduction for 2 out of 4 pacing cycles (e.g., 2 out of 4 A-A intervals), the algorithm performs a mode-switch to the DDD(R) mode. Pacing occurs in the DDD(R) mode for a period of time (e.g., 1 minute), after which the algorithm checks for the resumption of intact conduction, e.g., by switching to the AAI(R) mode for one atrial cycle to check/test for intact AV conduction. If intact AV conduction is not found (i.e., if the test for intact AV conduction fails, and thus it is determined that there is AV block), the MVP algorithm reverts to the DDD(R) mode for a further period of time (e.g., two minutes). Then, after the further period of time (e.g., two minutes), the algorithm again switches to the AAI(R) mode for one atrial cycle to again check/test for intact conduction. Each period of time during which DDD(R) pacing is performed can be referred to as the DDD mode pacing interval. The algorithm doubles the DDD mode pacing interval after each failed test (e.g., from 1 minute, to 2 minutes, to 4 minutes, to 8 minutes, etc.), up to a maximum of 16 hours, then tests every 16 hours thereafter.
When using the MVP algorithm, patients could become symptomatic because of frequent dropped beats caused when retesting for intact AV conduction. Further, at least some patients with an implanted cardiac device using the MVP algorithm have experienced other symptoms relating to the MVP algorithm. One such problem occurs when premature ventricular contractions (PVCs) cause retrograde conductions. More specifically, the MVP algorithm, when operating in the AAI(R) mode, may detect the retrograde events as P-waves. Then, since there is no R-wave afterwards (i.e., following retrograde P-waves) to detect, the device interprets this as skipped beats, and the device may mode switch to the DDD(R) mode. This has resulted in repeated mode switching between the AAI(R) and DDD(R) modes, which has caused patients to become symptomatic. Additionally, the retrograde P waves have caused the AEI interval to be reset, which causes long pauses in the ventricular rhythm (i.e., long pauses between consecutive R-waves), which may also play a role in the patients becoming symptomatic. It would be beneficial if such deficiencies of the MVP algorithm can be overcome.